A very important step in the papermaking process is screening of fiber pulp suspensions. Traditionally, the pulp suspension is screened by several so-called pressure screening apparatus of the type described above interconnected in a system of screening apparatuses, in which each screening apparatus represents a screening stage dependent on the other stages of the system.
As an alternative to the traditional screening system with several interconnected screening apparatus, one single screening apparatus may be designed with several stages, typically two or three stages, incorporated into the same screen body. A variety of such multi-stage screening apparatus of various designs have recently been introduced to the market.
The increasing size of the paper making production lines of today has resulted in very large screening apparatus. Especially screening apparatus for low consistency pulp suspensions is large and has a very large screen basket, in order to accommodate high hydraulic loads. The screen baskets for different screening apparatus are typically designed with about the same aspect ratio—length/diameter—regardless of size, so that a large basket is very long. Another reason why many screen baskets are long is the fact that it is considerably cheaper to increase the size of a given screen by increasing the length of the screen basket as compared to increasing the diameter thereof.
In a long screen basket the path of travel for debris particles will be long. As a consequence, a long screen basket has the disadvantage that since the retention time for the individual particle that is to be rejected will be long the probability of acceptance or breakdown will be higher than in shorter screen baskets. Furthermore, a long screen basket is likely to encounter problems with reject fraction thickening and will have lower capacity per unit surface area as well as reduced removal efficiency.
One way to counteract the reject fraction thickening is to dilute it with dilution liquid, typically water, and there are prior screening apparatus provided with arrangements to add dilution water to the inside of the screen basket for this purpose. For example, U.S. Pat. Nos. 6,080,274 and 6,186,333, and WO 00/50690 disclose expensive dilution water arrangements built into multi-stage screening apparatus. A serious disadvantage of these known multistage screening apparatus is the need for expensive hardware for process control in form of very large valves and flow meters on the accept lines from the different stages. Each accept compartment requires a separate flow control with flow meters and control valves.
Another known dilution arrangement includes revolving dilution water outlets integrated into the rotor. However, with this kind of dilution arrangement it is difficult to get the pressurized dilution water from the screen housing into the rotor. There are seals between stationary and rotary parts of the apparatus that often have wear problems, so that fibers pass through the seals into the dilution water compartments and eventually plug the outlets for dilution water. Another known dilution arrangement includes stationary dilution water outlets below the screening zone, and integrated into the screen housing. With these fairly expensive known arrangements it is very difficult to transport the dilution water to the optimum destination in the screen basket.
Swedish patent application No. 9601979-9 proposes a solution to the above noted problems and discloses a dilution arrangement in which dilution water is introduced into a channel circumventing a wedge wire type of screen basket. The channel is formed by putting a lid over the space between two support rings on the screen basket. The dilution water is fed into the screen basket through screening slots provided on the mantle wall of the screen basket. However, a problem with this solution is that the flow of dilution water entering the inside of the screen basket through the many very fine screen slots is insufficient and cannot provide for enough penetration and mixing of the dilution water and the thickened reject fraction. Another problem is leakage of unknown quantities of water to the accept chamber located external to the screen basket through the axially open spaces at the outer narrow ends between the wedge shaped bars and the fixation and support rings, that constitute the top and bottom of the dilution water channel.
One object of the present invention is to provide a screening apparatus for screening pulp suspension in stages having a simple, inexpensive dilution means that supplies dilution liquid to an optimum destination in the screen basket for efficient dilution of the reject fraction.
Another object of the invention is to provide a screen basket for use in the screening apparatus of the present invention and also for replacing worn out screen baskets in existing screening apparatuses.
Yet another object of the present invention is to provide a method of screening pulp suspension in stages so that the developed reject fraction is diluted in an optimum manner.
The initial object set forth above is obtained by a screening apparatus of the type described initially characterized in that the screen basket includes at least two separate tubular screen sections, and that the dilution means comprises at least one annular element axially interconnecting the two tubular screen sections and forming a tubular dilution liquid compartment extending at least substantially around the screen basket, the annular element forming a plurality of dilution liquid ejection passages between the dilution liquid compartment and one of the central chamber and outer chamber.
As a result, the required amount and velocity of the dilution liquid jets sprayed from the ejection passages to provide efficient dilution of the reject fraction is easy to achieve by properly designing the size of the ejection passages.